1. Field of the Invention
This invention relates to an attraction holding device for accurately holding a measurement object in an interferometer or the like.
2. Description of the Prior Art
Interferometers such as flatness testers for measuring the surface flatness of measurement objects are used for inspecting the surface flatness of silicon wafers used as a substrate of an integrated circuit prior to the manufacturing process of integrated circuits. To make accurate measurements, it is necessary to fix the measurement object on the stage of the interferometer without any distortion induced. Also when the surface of objects must be processed or machined precisely, it is required to securely hold the object without distorting or deforming the objects.
In order to satisfy the aforesaid requirements, there has heretofore been used a method in which the measurement object is held by attraction on a reference plane exhibiting a high flatness. Specifically, the measurement object is placed on the reference plane with suction holes perforated therethrough, and then the object is attracted by an evacuation with a vacuum pump through the suction holes, being securely held on the reference plane.
In such method as described above, it is important not only to process or machine the surface of the reference plane for holding the object to a flatness of the order of light wavelengths but also not to have the reference plane distorted or deformed when holding the object thereon. In case the object is held by an evacuation on the reference plane, there arises an air pressure gradient between the inside and outside of the reference plane. For this reason, when constructing attraction holding devices including the reference planes, it must be noted that distortions or deformations attributable to the air pressure gradient will be established in the reference plane.
Heretofore, although careful consideration has been given from the point of view of accurately machining and securely mounting the reference plane, there has not been thorough enough study of a manner of maintaining the accuracy of the surface of the reference plane on which the object is held by evacuation.